Device for protecting fibre lines against destruction by laser radiation

ABSTRACT

The invention relates to laser engineering and fibre optics. The inventive device for protecting fibre lines against destruction thereof by laser radiation is embodied in the form of a section of an optical fibre which comprises the cladding ( 6 ) and the core ( 7 ) thereof. The position ( 8 ) indicated dashed lines which show the position of the field of optical fibre mode. The distance between said dashed lines is equal to the diameter (D) of the field of light-guide mode. The position ( 10 ) indicates the section of the light-guide having the reduced diameter of a reflecting cladding. Said device operates as follows: a pressure of 10 4  atm. is produced by a high temperature in the core ( 7 ) during the propagation of an optical discharge wave. The pressure of 10 4  atm. is close to the strength limit of the optical fibre material heated by the optical discharge, that results in the stopping of the optical discharge wave. For this reason, the fibre lines are provided with at least one section of the light-guide, which has a reduced thickness fused quartz cladding and undeformed core.

The present invention relates to laser engineering and fibre optics andis applicable in optical communication systems, fibre laser devices fortreating materials, and in medicine and other fields where opticalradiation of the order of 1 Watt and higher of optical power istransmitted through optical fibres.

As of now, the information transmission velocity is drastically growingin optical communication systems.

The increased velocity of information transmission through opticalfibres necessitates a great number of channels to be carried by a singleoptical fibre, using the wavelength division multiplexing (WM). There isalso a growing demand to use optical amplifiers in optical communicationlines.

The increase of the number of channels transmitted over a single opticalfibre, the use of optical amplifiers (both erbium optical amplifiershaving a great output power, and Raman fibre amplifiers), and input ofoptical amplifier pumping light into an optical fibre even now lead toincrease of the average optical radiation power in the optical fibre upto the level of the order of 1 Watt.

At such power levels, the phenomenon of destruction of optical fires byoptical radiation may occur. This phenomenon in silica glass fibres (itis just such fibres form the basis of current optical communicationlines) is referred to as “catastrophic damage” or “fuse effect”.Actually, the optical fibre destruction by laser radiation resemblesburning of a blasting fuse.

In this description, the above phenomenon will be referred to aspropagation of a subsonic optical discharge wave (i.e. travelling at asubsonic velocity) through an optical fibre, which reflects the physicalnature of occurring processes, or, for brevity, as an optical dischargewave propagation.

Information about destruction of single-mode optical fibres by opticalradiation was first published in 1987 (Raman Kashyap, Self-PropelledSelf-Focusing Damage in Optical Fibres, Proc. Conf. Lasers '87, LakeTahoe, Nev., Dec. 7-11, 1987, pp. 859-866).

The above work substantially demonstrated that where a laser radiationof the order of 1 Watt of optical power propagates over an opticalfibre, there may appear an optical discharge wave; see FIG. 1 where 1 isan optical fibre, 2 is a plasma glow region in the optical dischargewave; 3 is the arrow showing the direction of laser radiation input tothe optical fibre, and 4 is the arrow showing the direction of opticaldischarge wave travelling through the optical fibre. But the authors ofthe aforementioned work interpreted the phenomenon in a different way.Externally it looks like the movement of a bright white or bluish glowregion (looking like a small “star”) through the optical fibre core,which propagates towards the laser radiation with a velocity of about 1m/s. The bright glow region is a low-temperature plasma region. The“star” temperature was estimated to be about 5400K on the basis of itsglow spectrum (see D. P. Hand, P. St. J. Russel, Solitary thermal shockwaves and optical damage in optical fibres: the fibre fuse. OpticsLetters, Vol. 13, No. 9, pp. 767-769, 1988).

But the process does not appear spontaneously when the above power isinput to an optical fibre. To initiate the process, it is required tocreate in the optical fibre, in which a radiation of the order of 1 Wattpower exists, a region of increased laser radiation absorption, e.g. byheating a section of the optical fibre up to about 1000° C. (in electricarc or by a burner), applying a laser radiation absorbing substance onthe optical fibre end face, or by bending the optical fibre at areasonably small radius. In particular, the process of optical dischargewave propagation can be initiated by touching the output end of theoptical fibre (the end from which laser radiation is output) with alight absorbing surface (including metallic one).

Actually, after initiating the process in the fibre core there isgenerated a dense plasma, which on one hand absorbs the radiationpropagating through the optical fibre, and on the other hand, transfersheat energy to surrounding cold material layers by thermal conductance.The so-heated core regions lying in the laser radiation field begin, inturn, to absorb the radiation. Therein lies the mechanism of opticaldischarge propagation through the optical fibre core, which is similarto the mechanism of propagation of slow chemical combustion front ingases and solids (L. D. Landau, E. M. Lifshitz, Hydrodynamics, Moscow,Nauka, 1986 (Chapter XIV, §128, Slow combustion, pp. 662-670). The term“slow” has here the meaning of “propagating with subsonic speed”.

Upon propagation of the discharge wave, cavities (or voids) of sizeabout several micrometers are formed in the optical fibre core in mostcases, wherein the cavities may generate a periodic structure along thefibre core (see FIG. 2). Formation of such cavities completely breaksdown the optical fibre light-guiding properties.

A method is known for protecting optical communication lines againstcomplete destruction when an optical discharge wave appears in the fibre(D. P. Hand, T. A. Birks, Single-mode tapers as ‘fibre fuse’ damagecircuit-breakers, Electronics Letters, vol. 25, No. 1, pp 33-34, 1989).

The method involves providing and employing unique circuit-breakersagainst propagation of an optical discharge wave through optical fibres.It is known that the process of optical fibre destruction by radiationsubstantially depends on the laser radiation intensity in the opticalfibre. To reduce the laser radiation intensity at least at some sectionof an optical communication line, the authors proposed to include intothe optical fibre line an optical fibre section having a waist, see FIG.3, where 5 is an optical fibre waist region; 6 is a reflecting claddingof the optical fibre, made preferably of silica, hatched in FIG. 3; 7 isthe optical fibre core; 8 is the dashed line showing boundaries of thelaser radiation mode field in the optical fibre and its change with thecore diameter; and 9 is the centreline of the optical fibre. As can beseen at FIG. 3, the optical fibre section with a waist is a section ofthe optical fibre, at a part of which the fibre cross-section diameteris reduced, all of the other dimensions defining the fibre cross-sectionstructure, e.g. the optical fibre core diameter, being also reduced inproportion. The waist in FIG. 3 can be considered as a combination oftwo series connected tapers. When light passes through the waist region,the mode field diameter in the optical fibre changes (increases), thisresulting in reduction of the radiation intensity and, under favourableconditions, arresting the optical discharge propagation process. Themode field diameter, often referred to as MFD, implies the valuecommonly used in fibre optics, which defines the lateral dimension ofthe spatial region occupied by radiation field in a single-mode opticalfibre, as it is defined e.g. in book by G. Keiser. Optical FibreCommunications. (Third edition) McGraw Hill, pp. 63-64, 2000 andreferences in it.

FIG. 5 shows a plot of the relationship obtained experimentally bymeasuring the dependence of the minimal laser radiation power in theoptical fibre core required to sustain the optical discharge wave onmode field diameter for optical fibres of different types:germanosilicate fibres (i.e. with the core containing mainly SiO₂ andGeO₂) and phosphosilicate fibre (i.e. with the core containing mainlySiO₂ and P₂O₅). Numerals near experimental points indicate the opticalfibre type: 1—phosphosilicate fibre, 2-7—germanosilicate fibres. Withincreasing the mode field diameter in the optical fibre, the thresholdpower value needed to sustain the optical discharge propagationincreases. The most pertinent prior art is based on this effect.

As the result, only a fibre section between the discharge waveinitiation point and arrest point is damaged, rather than the entireoptical fibre line. The authors emphasize that the method implemented ina device is suitable for only single-mode fibres, since just in suchoptical fibres with the waist included in the line the radiationintensity can be reduced. Therefore, the method arrests undesirableprocesses and protects against damage the communication line portionwhich lies between the described optical fibre section and the laserradiation source.

The prior art device for protecting optical communication lines suffersfrom the following basic disadvantages:

a) The optical fibre core parameters are changed in the device, whichcould result in additional losses and distortion of useful signal, e.g.due to partial reflection thereof;

b) As power increases, the radiation intensity increases in the opticalfibre core, including the waist region, this leading to uninterruptedpassage of the optical discharge wave through the waist region. Thus,the device loses its efficiency when the radiation power increases.

Disadvantages of the prior art device further include complexity of itsmanufacture, requiring that the optical fibre be heated and preciselystretched.

To provide unambiguous understanding of the matter of the presentinvention, the authors have introduced in the description a number ofdefinitions of optical fibre parameters. There has been introducedparameter d of the optical fibre cross-section. The cross-sectionparameter d of the optical fibre section under consideration is definedas a diameter of a fused quartz cladding of the optical fibre in thecross-section under consideration, if the optical fibre possessescylinder symmetry with respect to the axis, and the parameter d isdefined as double minimal distance from the geometric centre of theoptical fibre core cross-section to the fused quartz cladding boundary,if the shape of optical fibre does not possess cylinder symmetry (incase of cylindrical optical fibre, the parameter definition coincideswith the definition of d presented above as the optical fibre claddingdiameter).

In light of the above, the object of the present invention is to providea device for protecting fibre lines against destruction by opticaldischarge wave propagating through the optical fibre exposed to laserradiation, which would maintain its efficiency when the laser radiationpower increases, without introducing additional optical losses into theoptical communication line.

The above object is attained by providing a device for protecting afibre line against destruction by laser radiation, comprising a sectionof an optical fibre having a core with a constant diameter throughoutthe length of the section, and a cladding of the optical fibre section,said cladding having at least at one part of length L≧10·D of saidoptical fibre section a cross-section parameter d in the range D<d≦min(4D, 40 μm), where D is the mode field diameter.

The optical fibre cladding is made of fused quartz glass.

Said optical fibre section is formed directly in the fibre line to beprotected.

Said optical fibre section is further included into the fibre line to beprotected, e.g. by splicing or connecting by optical connectors.

Furthermore, said optical fibre section may be cylindrical, with thecore having a constant diameter throughout the length of said section,and the cladding diameter at least at one part of length L≧10·D of saidoptical fibre section being in the range D<d≦min (4D, 40 μm), where D isthe mode field diameter.

Said cylindrical optical fibre section can be formed directly in orfurther included into the fibre line to be protected, e.g. by splicingor connecting by optical connectors.

The device in accordance with the present invention employs a quitedifferent physical mechanism of arresting the optical discharge wavepropagation, namely, by reducing the laser radiation absorption factorin plasma of the optical discharge propagating through the optical fibrecore, with the plasma density being reduced.

The authors have found the phenomenon of deformation and even completedestruction of the optical fibre quartz cladding by mechanical andthermal exposure to plasma of the optical discharge propagating throughthe optical fibre exposed to laser radiation.

When the core substance is heated by the optical discharge wave up toabout 5000K with the substance volume unchanged, pressure in the plasmaregion reaches about 10 ⁴ atm.

On the surface of the quartz cladding of a standard optical fibre with125 μm diameter, temperature increases by 200÷300K only after theoptical discharge have passed through the core. As this takes place, theoptical fibre surface is under pressure of 1 atm. But as known fromexperiments, fused quartz glass fibres exhibit a reasonably highmechanical strength, which prevents destruction of the optical fibrecladding under such conditions, and this in turn creates conditions forpropagation of the optical discharge wave through the optical fibre.

If, however, a part of the fused quartz cladding is removed at a part ofthe optical fibre (e.g. by locally reducing the cladding diameter) sothat the remaining part of the cladding were heated by plasma of theoptical discharge wave to a temperature at which, under excessivepressure created by the plasma, deformation (expansion even up tocomplete mechanical distortion) of the fibre cladding would be possible,leading to plasma density reduction and, as a consequence, to reducedabsorption of laser radiation therein and respectively reduced energyrelease, the propagation of the optical discharge wave will cease.

The higher the laser radiation power, the greater is the pressure andtemperature in the optical discharge plasma and the faster the opticalfibre will be distorted and the optical discharge wave arrested. Thus,with increasing the laser radiation power, the reliability of the deviceis only improved (in contrast to the most pertinent prior art). Whilethe fused quartz cladding diameter is reduced, the optical fibre core isnot changed, this providing a small perturbation of waveguide channelparameters, again in contrast to the most pertinent prior art.

Dimensions of the optical fibre section with a reduced diameter of thequartz glass cladding are specified so that to provide a desiredsensitivity of the device and maximum allowable value of distortionsintroduced into waveguide properties of the optical fibre.

The authors conducted several series of experiments in order todetermine whether it is possible to arrest the discharge wavepropagation through the optical fibre. The experiments were conducted asfollows. An optical fibre section with a fused quartz cladding ofstandard diameter 125 μm was etched in hydrofluoric acid solution. Afteretching the cladding diameter reduced, the final diameter value beingcontrolled by the etching time. Radiation of a continuously operated,optical fibre laser with waves of different power was then input to theoptical fibre at one side. At the other end of the optical fibre, anoptical discharge wave was initiated, which propagated through theoptical fibre towards the laser radiation and passed through the sectionwith etched fused quartz cladding. In different experiments there wereused optical fibres with different core parameters, different values ofthe quartz cladding diameter in the etched region, and optical fibrelasers with different radiation wavelengths (from the series of 1.06 μm,1.24 μm and 1.48 μm). In every experiment, the optical discharge waveeither passed through the section with reduced cladding diameter, or theprocess of propagation was interrupted (see FIG. 8 and FIG. 9). FIG. 8shows a photograph of an operated device for protecting opticalcommunication lines, implementing the method in accordance with theinvention, wherein 11 are cavities or voids formed in the optical fibrecore region after passage of the optical discharge wave. Here, the shapeof the voids is different from that seen in FIG. 2. Numeral 12 shows theregion of destruction of the optical fibre fused quartz cladding byplasma pressure in the slow optical discharge wave. Scale of thephotograph is as follows: height of FIG. 8 is 65 μm, and width is 250μm. The laser radiation propagated from left to right.

FIG. 9 shows a photograph of a device for protecting fibre lines inoptical communication lines against destruction by laser radiation,according to the invention, wherein a is a view of the device beforeexciting the optical discharge wave in the optical fibre; b is a view ofthe device after operation, i.e. after arresting the optical dischargewave. The arrest area is encircled. The optical fibre was not damaged atoperation in contrast to that in FIG. 8. Scale of image at FIG. 9, a andb is as follows: each scale division in FIG. 9,b corresponds to 0.1 mm;FIG. 9, c is the enlarged image of the area approximately encircled inFIG. 9,b. Scale: full frame width corresponds to 1 mm. Laser radiationpropagated from right to left.

Despite the great number of parameters changing onexperiment-by-experiment basis (concentrations of various impurities inthe core, core diameter, difference between refractive indices of thecore and the cladding, radiation wavelength, optical fibre claddingdiameter in the etched region, radiation power input to the opticalfibre), the obtained results demonstrated that the process of arrestingthe optical discharge wave is defined by the diameter of radiation modefield in the optical fibre and the diameter of optical fibre cladding inthe region where it was reduced by etching.

Particularly, the experiments performed by the authors showed thatarresting of the optical discharge wave as it propagates through theoptical fibre with reduced diameter cladding (denote the diameter asparameter d of the cross-section of the optical fibre section inquestion) occurred in the case where the optical fibre claddingparameter did not exceeded the smallest of the values (4·D) and (40 μm),i.e. where d≧min (4D, 40 μm), where D is the diameter of the mode fieldin a single-mode optical fibre through which laser radiation propagates(before reducing the cladding dimensions). (In the fibre opticsbackground art the value is often referred to as mode field diameter,MFD). On the other hand, to restrict distortions of various type, whichcan be introduced by such narrowing into the radiation transmissionchannel, the smallest value of parameter d should be greater than D (seeFIG. 4).

Additionally, the same experiments showed that to halt the process ofoptical discharge wave propagation it is necessary that length L of theoptical fibre part with a reduced value of parameter d were no smallerthan 10·D, i.e. L≧10·D (see e.g. FIG. 9).

To arrest the optical discharge wave, another cross-section shapes(different from circular) of the optical fibre with reduced claddingdimensions can be also used. In such a case, parameter d is defined asdouble minimal distance from the geometrical centre of the optical fibrecore cross-section to the boundary of the fused quartz cladding (in caseof cylindrical optical fibre, the parameter definition coincides withthe definition of d given before as the optical fibre cladding diameterin the narrowing region). But as axially symmetric (or nearly axiallysymmetric) fibre elements are much easier to fabricate, only suchstructures are presented as examples.

Therefore, an inventive device for protecting fibre lines againstdestruction by optical discharge wave comprises an optical fibresection, wherein at some part thereof having length L a parameter d ofthe optical fibre is reduced to the values shown in FIG. 4 by hatchedarea. In so doing, the optical fibre core remains substantiallyunchanged, and the optical fibre waveguide properties suffer minimalchanges.

In the final analysis, a fibre line is protected by disposing therein atleast one optical fibre section with a changed cross-section shape, orby changing the cross-section shape of an optical fibre section in theline to be protected so that

a) the optical fibre core diameter remains unchanged, and

b) parameter d of the optical fibre in the part with changedcross-section is d≦min (4D, 40 μm), the part having length L≧10·D.

The inventions will be further explained by description of an exemplarypreferred embodiment with reference to the accompanying drawings,wherein similar elements are denoted by the same numerals throughout thedrawings.

FIG. 1 is a schematic diagram of propagation of an optical dischargewave through an optical fibre, according to the prior art;

FIG. 2 is a photograph of periodic structure of voids in the opticalfibre core (optical fibre LEAF, Corning), formed under exposure toradiation of Nd:YAG laser with 1.06 μm wavelength and 4.2 W power. Theradiation propagated from left to right. Scale: every division is 10 μm;

FIG. 3 is a schematic diagram showing implementation of a method forprotecting an optical fibre against destruction by laser radiation,according to the prior art;

FIG. 4 is a plot illustrating values of diameter d of fused quartz glasscladding, which provide functioning of protection elements according tothe invention, depending on the mode field diameter of the optical fibre(hatched region);

FIG. 5 is a relationship obtained experimentally by measuring thedependence of a minimum power of laser radiation in the optical fibrecore required to sustain the optical discharge wave propagation, foroptical fibres of different types: germanosilicate fibres (i.e. with acore containing mainly SiO₂ and GeO₂) and phosphosilicate fibre;

FIG. 6 is a schematic diagram illustrating a device for protecting anoptical fibre against destruction by laser radiation, in accordance withthe invention;

FIG. 7 is a cross-section of the device in accordance with the inventionat line A-A (see FIG. 6);

FIG. 8 is a photograph of operated device for protecting fibrecommunication lines, implementing the method in accordance with theinvention. Scale: height=65 μm, width=250 μm. Laser radiation propagatedfrom left to right;

FIG. 9 is a photograph of a device for protecting optical fibres inoptical communication lines against destruction by laser radiationaccording to the invention, wherein a is a view of the device beforeexciting the optical discharge wave in the optical fibre; b is a view ofthe device upon operating—arresting the optical discharge wave; c is anenlarged image of the region approximately shown by circle in FIG. 9,b.

Having been initiated in an optical fibre, optical discharge travelstowards laser radiation (see. FIG. 1), and if there are no obstacles onits path, the discharge will pass the entire distance through theoptical fibre to the radiation source. After the optical dischargepassage, the optical fibre loses its waveguide properties due to damagesuffered by the structure of the optical fibre core (see FIG. 2).

FIG. 6 shows a schematic diagram of a possible embodiment of a devicefor protecting an optical fibre against destruction by laser radiationin accordance with the present invention. The device for protecting anoptical fibre comprises a section of an optical fibre as shown in FIG.6, where 7 is an optical fibre core, 6 is an optical fibre cladding, 8are dashed lines showing the position of the mode field in the opticalfibre. The distance between them is equal to the diameter (D) of thelaser radiation mode field in the optical fibre. Numeral 10 shows theoptical fibre part with a reduced diameter of the reflecting cladding.FIG. 7 shows the cross-section along line A-A.

The device is included, e.g. by splicing, into a protected fibre lineand functions as follows. When an optical discharge wave propagatesthrough the optical fibre, as described before, pressures of about 10 ⁴atmospheres are produced by high temperature in the core 7 region. The10 ⁴ atm. value is close to the destruction limit of the optical fibrematerial heated by heat conductance and radiation from the plasma regionup to temperatures of several hundreds degrees, therefore, at a quitethin optical fibre cladding this phenomenon leads to optical fibredestruction, pressure drop in the core region, drastic decrease indensity of the plasma absorbing laser radiation, respective reduction ofradiation absorption, and, as consequence, to arresting the opticaldischarge wave.

Therefore, at least one optical fibre section with a reduced thicknessfused quartz cladding 6 is provided in fibre lines, while the core 7 isabsolutely non-deformed as shown in FIG. 6. This operation, on one hand,does not introduce significant changes into the signal propagationchannel (the core is not deformed), and on the other hand, suchprotecting devices are easy to fabricate: the optical fibre claddingdiameter can be reduced by simple etching.

An implemented embodiment of a device for protecting optical fibresagainst destruction by laser radiation is shown at photograph of FIG. 9.The device comprises an optical fibre section with the mode fielddiameter of 8.9 μm, the external fibre diameter being reduced at a part(in this case having 1 mm length) by etching the part in HF acidsolution. Laser radiation propagated from right to left. FIG. 9,a showsthe device before operation. Radiation having a power of up to 5 Wpassed through it unimpeded. FIG. 9,b shows the same device afterinitiating outside thereof an optical discharge wave, which propagatedfrom left to right through the optical fibre. As the wave has reachedthe optical fibre location where the cladding diameter was about 30 μm,its propagation ceased. Minimum diameter of the optical fibre in thedevice shown at FIG. 9 was 20 μm.

To the left of the arrest point (shown by circle at FIG. 9,b) theoptical fibre destruction character is the same as in the optical fibrewith a normal fused quartz cladding diameter—a periodic sequence ofvoids in the optical fibre core. Only a short distance before the arrestpoint, the voids increase in size, and the optical discharge propagationprocess ceases. The discharge propagation arrest is accompanied eitherby expansion of the optical fibre section, as in FIG. 9,b,c, or bydestruction of the optical fibre in the arrest point (FIG. 8).

A device for protecting fibre optical fibres can be also implemented byforming an optical fibre section with a reduced diameter of fused quartzcladding immediately at the fibre of the optical line to be protected,rather than on a dedicated section of the optical fibre, which is thenspliced into the optical fibre line to be protected. In this case thereis not need additional splicing of optical fibres, which leads toadditional in-line losses of radiation, but implies in-the-fieldfabrication of respective narrowings in optical fibres, this having itsnegative effects.

To protect long-distance telecommunication lines, such protection deviceshould be periodically disposed along the line so that only one periodfails where an optical discharge wave unexpectedly appears in thetelecommunication network.

The device in accordance with the invention is applicable intelecommunication lines specifically, for protecting optical fibres fromdamage by laser radiation. The device can be further used in processesof laser treatment of materials, in laser surgery and other medicallaser systems for protecting the emitting laser.

1. A device for protecting a fibre line against destruction by laserradiation, comprising a section of an optical fibre having a core with aconstant diameter throughout the length of said section, and a claddingof said optical fibre section, said cladding having at least at one partof length L≧10·D of said optical fibre section a cross-section parameterd in the range D<d≦min (4D, 40 μm), where D is the mode field diameter.2. The device according to claim 1, characterized in that said opticalfibre cladding is made of silica based glass.
 3. The device according toclaim 1, characterized in that said optical fibre section is formeddirectly in the fibre line to be protected.
 4. The device according toclaim 1, characterized in that said optical fibre section is furtherincluded into the fibre line to be protected, e.g. by splicing orconnecting by optical connectors.
 5. The device according to claim 1,characterized in that said optical fibre section is cylindrical, withthe core having a constant diameter throughout the length of saidsection, and the cladding diameter d of the optical fibre section atleast at one part of length L≧10·D of said optical fibre section beingin the range D<d≦min (4D, 40 μm), where D is the mode field diameter. 6.The device according to claim 5, characterized in that said opticalfibre section is formed directly in the fibre line to be protected. 7.The device according to claim 1, characterized in that said opticalfibre section is further into the fibre line to be protected, e.g. bysplicing or connecting by optical connectors.